Certain types of electrical equipment, for example, motors and other devices, can be configured to receive electrical power from multiple alternate sources. One common configuration is an AC motor that is powered by either the utility AC power lines, or by a variable frequency drive. In common applications, the variable frequency drive is used as a primary source of electrical power because it can be used to vary the speed of an AC synchronous motor. However, utility AC power is provided as a back-up in the event that the variable frequency drive has a fault or otherwise should not be used.
In such situations, it has been a common practice to employ bypass relays and circuits that operate to connect the variable frequency drive to the input of the motor (or other electrical equipment) during normal operation, and then disconnect the variable frequency drive and connect the utility AC power to the input of the motor under certain “bypass” conditions. Examples of bypass conditions include a fault in the variable frequency drive, or an emergency condition in the building or facility in which the motor is disposed. The bypass operation may be triggered automatically, as by detection of an overload condition or the like within the variable frequency drive, or triggered manually, as by a pushbutton or other type of switch.
Early bypass systems were hardware intensive, and provided little or no intuitive information as to their operation to an observer. Sometimes little or no indication was provided as to the state of various system relays. Because of the lack of information, it could be difficult determining how the bypass system was operating, much less why the system was in a particular state or mode. Moreover, the electrical circuits of the early bypass systems required a relatively large number of assembled parts and provided limited functionality.
More recently, bypass systems have increasingly employed electronic circuit to address some of the drawbacks discussed above. For example, U.S. Pat. No. 6,316,896 to Karl Tikkanen et al. shows a bypass system that includes a variable frequency drive, and which includes a pushbutton interface and automated bypass operation. The push button interface also identifies the status of the output relays (AC power or variable frequency drive) and thus provides an indication of the source of power that is being provided to the electrical load. The device employs a microcontroller, which to some degree allows for enhanced functionality.
While the device described in U.S. Pat. No. 6,316,896 provides some additional functionality and status information, there is a need for further information regarding the operation of a bypass device. There is also a need for a bypass device that has a reduced part count in relation to the functionality provided.